From a navigational standpoint, an important difference
between hatchling turtles and older turtles is that hatchlings remain within broad
oceanic regions but do not navigate to highly specific geographic sites.
In contrast, juvenile and adult turtles in inshore habitats often exhibit strong
fidelity to particular feeding areas and return to such locations after long
migrations and experimental displacements. An ability to pinpoint specific
geographic targets is also evident during reproductive migrations in which adult
female turtles travel hundreds or thousands of kilometers to lay their eggs in
specific stretches of beach.

How sea turtles navigate to specific geographic targets
has remained an enduring mystery of animal behavior. Such an ability
implies that turtles possess both a compass sense for maintaining headings and a
positional or "map" sense to determine their position relative to a goal.
Many animals possess diverse compasses based on stars, the position of the sun,
patterns of skylight polarization, and the Earth's magnetic field. Until
recently, however, little was known about the mechanisms that underlie the
map sense in sea turtles and other migratory animals. In this series of
web pages we describe a recent experiment providing the first evidence that the map of
sea turtles is based at least partly on information derived from the Earth's
magnetic field.

Homing in Sea Turtles

Research on sea turtle navigation began with the writings and
musings of Archie Carr, a professor of zoology at the University of Florida, who
was among the first to take a scientific interest in sea turtles. In 1956, Carr
published a book titled The Windward Road, which detailed his travels
throughout the Caribbean. In a chapter called "The Captains", he described
conversations with turtle fishermen in the Cayman Islands, including a surprising anecdote told to him by a captain of a turtle boat.
The incident involved green turtles that were captured in their feeding grounds
along the coast of northern Nicaragua (see map below). The turtles were
branded with the initials of the fishermen, as was the custom at the time, and
then loaded onto a boat bound for market in Key West, Florida. As the boat
neared the Florida Keys, a violent storm struck. The boat capsized and the
turtles escaped back into the ocean. Some months later, turtle fishermen
working in northern Nicaragua were astonished to recapture two of the turtles
back in the same location where they had been caught initially. The
turtles could be identified unambiguously because the initials of the fishermen
were still clearly visible on their shells. Somehow the turtles had
returned home from more than 1000 km (600 miles) away!

This incident and others like it convinced Carr that sea
turtles have remarkable navigational abilities. In The Windward Road,
Carr wrote: "It is safe to suppose that green turtles do in fact have some sort
of extra sense ... that lets them
make long, controlled journeys in trackless seas." Subsequent experiments by Carr and by others confirmed that sea turtles are indeed capable of
navigating to
specific geographic targets after long migrations or experimental displacements.
For example, it is now known that if a juvenile turtle is captured in its feeding
area and released some distance away, the turtle is likely to home to the
same area where it was captured originally (Ireland, 1980;
Avens et al., 2003). A recent study also demonstrated that, if juvenile turtles are
captured in their feeding sites and transported to a distant site, the
turtles will attempt to swim in the direction of home even if placed into an arena where they cannot make
progress toward the goal (Avens and Lohmann, 2004).

Although the ability of juvenile and adult turtles to return to specific geographic areas has been well-documented, how turtles navigate to specific locations remained a mystery for many years.